a) Field of the Invention
The present invention belongs to the field of photoinduced gratings in optical media, and more particularly relates to a method and an apparatus for photoinducing gratings having a spatially variable and controllable diffraction efficiency and average local index change.
b) Brief Description of Prior Art
It is now well established that Wavelength Division Multiplexing (WDM) systems using Erbium Doped Fiber Amplifiers (EDFA) will be the next enabling technology to access the huge optical fiber bandwidth. In those type of systems, all-fiber wavelength selective devices such as bandpass filters, gain flattening filters for EDFAs, dispersion compensators, and filters with any spectral shape and fine tuning of the nominal wavelength of Bragg gratings will be required.
Photosensitivity in optical fiber can be used to fabricate wavelength selective devices, since it allows to change the refractive index in the core of the optical fiber. This is done by illuminating the core with UV light. Such refractive index change is permanent and can be successfully used to fabricate Bragg gratings to act as bandpass filters, chirped Bragg gratings to make dispersion compensators, and spectrally designed all-fiber filters.
FIGS. 1a to 1f (identified as "prior art") illustrate various types of modulated refractive index changes and their resulting reflectivity responses. It is well known that Bragg gratings having a uniform index modulation 7, as shown in FIG. 1a, exhibit sidelobes 9 on both sides of the main reflection peak 11 (FIG. 1b). Those sidelobes 9 are undesirable because they induce crosstalk between adjacent channels in WDM systems. It has been shown that those sidelobes can be suppressed if the coupling efficiency varies spatially along the grating length, as illustrate in FIGS. 1c and 1d. This operation, called apodization, is ideally achieved by photoimprinting an index change amplitude modulation 13 that has a bell-like shape along the grating length. However, such apodized gratings present a fine structure 15 on the short wavelength side of their reflection response and lead to an undesirable chirp of the Bragg wavelength. The variation of the average index change causes the local Bragg wavelength at the center of the grating to be longer than the local Bragg wavelength at both ends; the grating then acts as a Fabry-Perot cavity. In order to get rid of those short wavelength resonances, the average refractive index has to be compensated so that the Bragg resonance is uniform along the whole grating length. FIGS. 1e and 1f show an example of the shape such a refractive index change 17 might take, and the resulting dispersion-free reflective response.
A number of techniques have been developed to produce gratings having a constant refractive index change. For example, a double exposure method is disclosed in MALO, B. et al.,"Apodised in-fiber Bragg grating reflectors photoimprinted using a Phase Mask", Electronics Letters, vol 31, no 3, pp. 223-225 (1995). As implied by its name, this technique requires that the optical medium be exposed to the writing light twice: once to produce the modulated refractive index change, and another time with a shadow mask to compensate for nonuniform variations in the average index. Phase masks with a locally varying diffraction efficiency have also been developed (see ALBERT, J. et al.,"Apodisation of the Spectral Response of Fiber Bragg Gratings using a Phase Mask with Variable Diffraction Efficiency", Electronics Letters, vol 31, no 3, pp 222-223 (1995)), and systems to move the fiber and phase mask during the exposure have been proposed for pure apodization. In one such system described in COLE, M. J. et al., "Moving Fiber/Phase Mask-Scanning Beam Technique for Enhanced Flexibility in Producing Fiber Gratings with Uniform Phase Mask", a small dither is applied to the optical fiber while the phase mask is kept fixed. If the magnitude of the dither is half of a grating pitch, the net result is a DC index change. If apodization is required, the magnitude of the dither is changed accordingly along the exposed fiber length. However, because the fiber is dithered, this writing system is very sensitive to external perturbations.
Although the above-mentioned techniques usually produce very good apodized gratings, they require some post-processing, sophisticated phase masks or elaborate setups, making them unpractical for large-scale production. There is therefore a need for a simple and flexible technique to photoinduce apodized Bragg gratings or spectrally designed filters in optical fibers or other types of optical waveguides, while allowing to control at will the average index change over all the UV exposed region.